Dendritic cells are cells of characteristic shape, density, and surface properties found in small number in lymphoid and other organs. Their importance lies in the fact that they have a heavy representation of Ia molecules on their surface (molecules believed to be important in allowing antigen to be recognized by lymphocytes on the surface of antigen-presenting cells) and that they are extremely potent stimulators to lymphocytes in vitro. Dendritic cells have been isolated in our laboratory from lymph nodes using established techniques. At present, methods are being developed to obtain larger numbers of cells using the spleen so that studies of their surface receptors can be carried out.